Abbreviations:    3GPP—3rd Generation Partnership Project (standards body)    A-DPCH—Associated Dedicated Physical Channel    DL—DownLink    FDD—Frequency Division Duplex    GGSN—Gateway GPRS Support Node    GPRS—General Packet Radio System    GSM—Global System for Mobile Communications    HSDPA—High Speed Downlink Packet Access    HS—PDSCH High Speed Physical Downlink Shared CHannel    LLC—Logical Link Control    MAC—Medium Access Code    MS—Mobile station    PDP—Packet Data Protocol    RLC—Radio Link Control    RNC—Radio Network Controller    SGSN—Serving GPRS Support Node    SNDCP—Sub Network Dependent Convergence Protocol    TCP—Transport    TDD—Division Duplex    UL—UpLink    UMTS—Universal Mobile Telecommunications System    UTRAN—UMTS Terrestrial Radio Access Network    WCDMA—Wide band Code Division Multiple Access
The 3rd Generation Partnership Project (3GPP) specification is a standard for the third generation mobile telephony system. The system supports different user data rates for different users. The transmission power used for a certain user is determined by interference level in a certain cell, user data rate, channel quality and requested quality of the data transmission in the cell.
Control channels and traffic channels are referred to as logical channels. These logical channels are mapped onto physical channels.
A physical channel can be a radio broadcasting frequency, a pair of frequencies (including duplex separation) in an analog mobile system or a time slot on a pair of frequencies in a digital mobile system.
A physical channel may, for example, be a time slot on a particular frequency channel in a TDMA system and may also be said to be a carrier of a logical channel, such as a traffic channel (sometimes abbreviated TCH), or several logical channels, such as various control channels in a multi frame structure.
In digital systems based on TDMA the mapping process means that a time slot and a frequency are allocated to the traffic burst.
The traffic channels convey voice or data (such as Internet traffic) between the mobile and base station. The channels are allocated on a per call basis. Ordinarily, a time slot is used for one call only (full rate), but an alternative with two calls per time slot (half rate) has also been specified.
All the present mobile systems are based on multiple access, which means that all users have simultaneous access to the medium. This requires a number of rules to prevent situations in which mobiles “all talk at the same time”. The medium, which is a common resource, must satisfy the needs of all users.
Traffic channels are assigned through control-channel signalling.
Each cell in a cellular system can be regarded as an individual medium because a mobile leaving the cell loses contact with that cell's base station. However, all mobiles in the cell utilise the same radio resource, namely, that portion of the frequency spectrum which has been assigned to the cell.
One technique for channel multiplexing using multiple access is CDMA (Code division multiple access). CDMA systems are capable of transmitting and receiving over the entire frequency band and a “third dimension” is used to separate traffic channels, namely coding.
A typical feature of the CDMA technique is that all mobiles in the network are assigned a unique code: a chip sequence. When a mobile wishes to transmit a bit stream, it replaces every bit with its code (for ones) or with the code's ones complement (for zeros). The result is that a number of air interface bits (referred to as a “chip”), depending on the spreading factor used, are transmitted for every payload bit. Provided that the same modulation methods are used as those used in TDMA, the required bandwidth will be correspondingly larger. Instead of using some tens of kHz for a voice channel, the chip will be modulated over approximately 3.84 MHz. That is why this technique is also referred to as a spread-spectrum technique.
The mobile telephone system may, for example, be a WCDMA system which has a downlink transport channel called High Speed Downlink Shared Channel (HS-DSCH) mapped onto a set of physical channels called High Speed Physical Downlink Shared CHannels (HS-PDSCH). The HS-DSCH provides enhanced support for interactive, background, and, to some extent streaming radio-access-bearer (RAB) services in the downlink direction.
In a HSDPA supporting WCDMA system (e.g. according to 3GPP release 5) every user that is given access to a HS-PDSCH (High Speed Physical Downlink Shared Channel) channel must also have an associated dedicated Physical Channel A-DPCH used, for example, for communicating control information.
A typical action in case of overload (congestion) consists of the Radio Network Controller RNC releasing certain channels to reduce the load on the particular resource that is in shortage, for example DL Power, UL interference, DL Codes, etc. It is possible that also A-DPCHs will be released, especially if all interactive/background traffic is carried on HS-PDSCH and there's a need to make more DL Codes available to handle a sharp increase in conversational traffic (speech, video, . . . ) or if it turns out that A-DPCHs consume a significant amount of power.
Current congestion control randomly selects the A-DPCHs to be released for certain mobile stations and thereby inhibiting the downlink information flow in the HS-PDSCH for the thereby randomly chosen mobile station. This can lead to a situation where a user that is almost done with a download is thrown out of the system at the expense of a user that has just got into the system. As an end user, this can be very annoying, especially if the application gives some kind of indication of the progress of the download (one case could be that the end user can see that the down load is 95% completed and then interrupted).
Therefore, there is a need for an improved congestion control in a HSDPA based system or the like, with enhanced data transmission flow control giving more pleased users.